In the Pratchett Discworld:-Edit
Horace Pennysmart is indeed professor of Extreme Horticulture at Unseen University. In The Science of Discworld: Darwin's Watch, he comes to grief by stepping on an Australian jellyfish of the sort that can turn a whole leg an interesting shade of purple.
In the Pesimal Discworld:-Edit
He has a cameo role in the story Murder Most 'Orrible as an advisor in potentially lethal exotic plants. He also has cameos in Zoo Tales and Hear Them Chatter on the Tide as the Zoo's advisor on vegetarian diets for those really picky eaters.